I really like the ideas in this thread.
I'm a complete voidcritter fanatic. I had twin boys who collected the cards and trained them. One of them got his Yorier to level 10 which I'm really proud of. Unfortunately they are teens now and I think one of their future children will have to try and complete that collection.
My vampire twins do frog breeding. They already got the full collection, but they love keeping the frogs in their bedroom and breeding them for plasma packs.
I really think I should pay even more attention to collections and making them into mini hobbies!